Civil Engineering Reference
In-Depth Information
13.2.1 Semiconductor activation
The initial process for semiconductor-supported heterogeneous photoca-
talysis of organic and inorganic compounds is the generation of electron-
hole pairs, initiated by light absorption with energy equal to or greater than
the band gap (Fig. 13.1). The process under irradiation can therefore be
divided in four steps, as described by Schiavello (1988):
1. absorption of light, followed by the separation of the electron-hole
couple,
2. adsorption of the reagents,
3. redox reaction,
4. desorption of the products.
Photoinduced charge transfer to other species (organic or inorganic
molecules, water) relies on molecules migration and adsorption onto the
semiconductor surface.
At its surface, the semiconductor can donate electrons to reduce an elec-
tron acceptor, given that the semiconductor CB bottom must be higher than
the acceptor reduction energy, as a necessary condition. The acceptor in
most cases is represented by oxygen, which forms a superoxide ion, O
2
•−
, or
hydrogen peroxide, H
2
O
2
: they both have excellent reactivity and play
important roles in photocatalytic reactions.
On the other side of the band gap, holes can combine with electrons
provided by donor species adsorbed on the semiconductor surface, thus
oxidizing the donor itself: in this case, the necessary condition is that the
top of the semiconductor valence band must be lower than the donor oxida-
tion energy. Typical oxidation reactions take place between semiconductor
and water molecules to form hydroxyl radicals OH
•
, which are extremely
active and tend to react easily due to their strong oxidizing power.
A
adsorption
2
3
OXIDATION
e
-
1
CB
A
-
desorption
1
4
E
g
h
ν
VB
D
adsorption
2
h
+
3
REDUCTION
D
+
desorption
4
13.1
Schematic representation of photoactivation mechanism: h
υ
=
incident radiation energy, E
g
=
semiconductor bandgap, A
=
acceptor,
D
=
donor. Phases are numbered as in the related text.
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